Cct. Models for semiconductor diodes Diode Models Large SignalSmall SignalPiecewise Linear Ideal...

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Cct. Models for semiconductor diodes Diode Models Large Signal Small Signal Piecewise Linear Ideal Diode Offset Diode

Transcript of Cct. Models for semiconductor diodes Diode Models Large SignalSmall SignalPiecewise Linear Ideal...

Cct. Models for semiconductor diodes

Diode Models

Large Signal Small Signal Piecewise Linear

Ideal Diode Offset Diode

Offset Diode Model

Internal resistance =0

Cutoff frequency (c)>0 (Vγ=0.6 volts for Silicon)

Reverse current (Is) =0

Can be modeled as Ideal diode in series with Vγ

I-V characteristic curve

Offset diode accounts for the offset voltage (better approximation).

If VD < Vγ it will be OFF (open circuit in series with Vγ.

If VD> Vγ it will be ON (short circuit series with Vγ).

Given offset diode with V = 0.7V, Calculate ID.

we assume it is ON and then check current direction if it is consistent.

By inspection it is clear that current is consistent with our assumption

4 0.70.825

4

PSV VI

RV V

I mAk

Example 1

Example (cont.) Try solving assuming

that diode is off. Note that offset diode

voltage is not similar to the open cct. voltage.

Example 2

If VB = 2 V; R1 = 1 k; R2 = 0.5 K ; Vγ = 0.6 V. Determine Minimum value of V1 so D1 conducts.

Sol: we assume diode OFF.

Cct. Models for semiconductor diodes

Diode Models

Large Signal Small Signal Piecewise Linear

Ideal Diode Offset Diode

Large Signal Vs. Small signal Models

In small signal model, the exact I-V char. Curve is used to determine diode voltage and current

it is used to determine accurate operating point of the diode.

Large signal models use approximating I-V char. Curve and thus approximate results of diode voltage and current.

Small Signal Model

No approximation is used. Diode cannot be replaced

with linear element (open or short).

We need to deal with the non-linear properties of the diode.

KVL, KCL can be applied Nodal and Mesh analysis

cannot be used. Superposition only deals

with Linear circuit elements!!

)1( T

D

V

v

sD eII

The Operating point of the Diode

The operating point (or Q-point) is the point that determines the exact value of diode voltage and diode current.

Load-Line Analysis of Diode Circuits

equations. KVL or KCL writeto difficult is It

1Vn

vexpIi: diode a is there whenbut

,...dt

diLv ,

dt

dvCi iR,v use can We

T

DsD

Analysis" Line-Load"

the perform can we

given, is diode the

of curve V-I the If

viRV

:gives KVL

shown,circuit the For

DDSS

Example 1- Load-Line Analysis

point)-(Q point operating the at

voltage and current diode the :Find

diode the of curve V-I the

,Ω1kR 2V,V :Given

shown,circuit the For

SS

mA 1.3i V, 0.70V

point operating the at

analysis line-load perform

vi 10002

i.e., ,viRV

DQDQ

DD

DDSS

Example 2 - Load-Line Analysis

point operating the at

voltage and current diode the :Find

diode the of curve V-I the

,k 10R V, 10Vss :Given

shown,circuit the For

mA 0.93i V, 0.68V

point operating the at

analysis line-load perform

vi 10k10

i.e., ,viRV

DQDQ

DD

DDSS

Load-Line Analysis of Complex Circuits

What if we have more than one source and one load ?? Use the Thevenin Equivalent

Example 3

SS L

L S

Given: diode I-V curve, V 24V, R 1.2k , R 6k

Find: the load voltage v and source currents I

Applying Thevenin Equivalent

20 ,

1

- 0 ( _ _ _ )

20 1000( ) 0

2020 ( )

1000 1000

LT SS

L

LT

L

T T D D

D D

DD D

RV V V

R R

R RR k

R R

V R i v DC Load Line Equation

i v

vi v mA

20 ( )D Di v mA

L DQ

DQ

S SS L

v v 10.0 V

I 10

I (V -v )/R 11.67 mA

mA

Example 3 (cont.)

The Q point (Operation point) is the solution of the system.

(IDQ, VDQ)

Determine the operating point of the diode shown in the following circuit.

The I-V char. Curve is given below.

Example 4

Example 4 (cont.)

- 0

2 48.3( ) 0T T D D

D D

V R i v

i v

DQ

DQ

v 1 V

I 21mA

Cct. Models for semiconductor diodes

Diode Models

Large Signal Small Signal Piecewise Linear

Ideal Diode Offset Diode

Piecewise linear Diode Model For this model we

assume the following:

Cutoff frequency >0

Internal resistance (rD) >0 (but constant!!)

Reverse current =0.

The approximation mainly in the value of the internal resistance.

Piecewise-Linear Diode Models

Now, diode voltage is the sum of resistance voltage and cutoff frequency voltage.

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